1. Field of the Invention
The present invention relates to a thin film field effect transistor in which an amorphous oxide semiconductor is used for an active layer.
2. Description of the Related Art
In recent years, flat panel displays (FPDs) have been put to practical use, due to the progress made in liquid crystal and electroluminescence (EL) technologies, etc. Especially, an organic electroluminescence element (hereinafter referred to as “organic EL element” in some cases) formed using a thin film material which emits light by excitation due to application of electric current can provide light emission of high brightness at low voltage, and thus is expected to achieve reduction in device thickness, weight, and size, and power saving, etc. in wide ranging applications including mobile phone displays, personal digital assistants (PDA), computer displays, car information displays, TV monitors, and general illumination.
These FPDs are driven by an active matrix circuit including thin film field effect transistors (“thin film transistor”, hereinafter also referred to as “TFT”) each using, as an active layer, an amorphous silicon thin film or a polycrystalline silicon thin film provided on a glass substrate.
On the other hand, to make the FPD thinner, lighter, and more resistant to breakage, attempts are being made to use a resin substrate which is light in weight and flexible instead of the glass substrate.
However, fabrication of the transistors using the silicon thin films described above requires a thermal treatment process at a relatively high temperature, and it is difficult to form the transistors directly on a resin substrate which is generally low in heat resistance.
Hence, such TFTs have been actively developed using, as a semiconductor thin film, a film of an amorphous oxide, such as an In—Ga—Zn—O-based amorphous oxide, which can be formed at low temperatures. As the TFT which employs an amorphous oxide semiconductor can be formed at room temperature, the TFT can be fabricated on a film. Therefore, amorphous oxide semiconductors have been attracting attention as a material for active layers of TFTs for a film (flexible substrate) lately.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2007-73701 discloses a TFT having an electric insulating layer as a resistance layer, which contains an amorphous oxide and has an electric resistance of 1011 Ωcm or more, between an active layer and a gate insulating layer. JP-A No. 2007-73701 discloses that the resistance layer has an effect of decreasing an OFF current and a gate leak current. In view of revealing the effect, a thickness of the resistance layer is described to be preferably from 1 nm to 200 nm.
JP-A No. 2008-42088 discloses a thin film device provided with a laminated structure having a first insulator, an active layer containing an oxide semiconductor, and a second insulator in this order, and also having a first interface layer located at the interface of the active layer with the first insulator and a second interface layer located at the interface of the active layer with the second insulator, in which oxygen vacancy densities of the first interface layer and the second interface layer are lower than the oxygen vacancy density of a bulk of the active layer. Considering that the structure is achieved by subjecting the active layer, the first insulator, and the second insulator to oxidation treatment without being exposed to the air during the formation thereof, the metal compositions of the first interface layer and the second interface layer are the same as that of the active layer, and there is a difference only in the oxygen vacancy density. It is disclosed that, by controlling an oxygen vacancy density of the interfaces, an ON/OFF ratio of a drain current is improved.
JP-A No. 2007-123702 discloses a TFT in which an oxide semiconductor layer of such as In2O3 and an intermediate oxide layer containing Ga2O3 are laminated to form an active layer. In the structure, the intermediate oxide layer is so thin that a tunnel effect is revealed and is laminated with the oxide semiconductor layer. A TFT employing a plurality of these laminated structures as the active layer is disclosed. It is disclosed that, by providing the intermediate oxide layer, oxygen defects of the oxide semiconductor are prevented.